1. Technical Field
The present invention relates to spindle motors and disk-drive devices utilizing the spindle motors; in particular to low-profile spindle motors furnished with hydrodynamic bearings, and to disk-drive devices utilizing the spindle motors.
2. Description of Related Art
In hard-disk drives that drive hard disks and like recording disks, spindle motors utilizing hydrodynamic bearings that, in order to support the shaft and sleeve as either one rotates relative to the other, employ the fluid pressure of a lubricating fluid such as oil interposed between the two are known.
With regard to spindle motors utilizing hydrodynamic bearings of this sort, the applicant in the present application has proposed, in Japanese Laid-Open Pat. App. No. 2000-113582, a spindle motor as illustrated in FIG. 1. Between the bottom face of a rotor 100 and the top-end face of a sleeve 102 in the spindle motor depicted in FIG. 1, a thrust bearing section 104 is configured. Likewise, between the outer circumferential surface of a shaft 106 furnished integrally with the rotor 100, and the inner circumferential surface of the sleeve 102, radial bearing sections 108, 108 are configured. The thrust bearing section 104 generates lifting force on the rotor 100, and the radial bearing sections 108, 108 function to center-balance in the radial direction, and prevent wobble in, the rotor 100.
The spindle motor depicted in FIG. 1 makes the thrust plate that would be a component of the thrust bearing in conventional hydrodynamic bearings unnecessary. The consequent advantage is a simplified structure that reduces the cost of the motor and at the same time enables it to be slimmed, without appreciably compromising the bearing rigidity. Nevertheless, with the advent of the application of disk drives in miniature devices such as portable information terminals, demands are on the rise to make the spindle motors used in the disk drives even slimmer. In addition, calls for lowering the cost of spindle motors still further have gone hand in hand with reducing the cost of disk drives.
Running counter to this is the fact that in its sleeve 102 the spindle motor depicted in FIG. 1 is provided with a communicating passage 110 made up of a through-hole 110a and channels 110b, 110c. The communicating passage 110 brings outside air into the bearing areas—that is, it enables air to circulate into and out of the bearing areas—and thus would expose the end portions of the radial bearing sections 108, 108 to the air. Due to the pumping action of dynamic-pressure-generating grooves formed in each bearing section, areas in which the internal pressure of the oil retained among the bearing sections becomes negative, i.e., at pressure less than atmospheric pressure, arise. Upon a decrease in the internal pressure of the oil to a negative pressure level, air that is entrained in the oil during the process of charging the bearing sections with oil, or that is present due to being swept in by the dynamic-pressure-generating grooves, appears in the form of bubbles. The volume of the bubbles expands with increasing temperature or decreasing external environmental pressure. The volume expansion of the bubbles brings leaking oil toward the exterior of the bearing sections and impairs the spindle motor's durability and reliability. Furthermore, the dynamic-pressure-generating grooves that are formed in the bearing sections come into contact with the bubbles, which causes vibrations and worsens non-repeatable run-out. The rotational precision of the spindle motor therefore worsens. Accordingly, the spindle motor configuration includes the communicating passage 110 in order to exhaust bubbles to the exterior of the bearing sections.
To bore the communicating passage 110 for discharging bubbles in this way a drilling tool is used. The drill bit can only be so small, however, to be strong enough for machining, which limits how small the through-hole 110a and the channels 110b, 110c that constitute the communicating passage 110 can be made. Consequently, the axial dimension of the shaft 106 and the sleeve 102 must necessarily be at least a given size for boring the communicating passage 110 and be extensive enough to maintain bearing rigidity in the radial bearing sections 108, 108. These requirements stand in the way of making the spindle motor slimmer.
What is more, the fact that the through-hole 110a as well as the channels 110b, 110c that constitute the communicating passage 110 are formed in the sleeve 102 complicates that part of the structure and at the same time increases the number of manufacturing processes. An increased-cost spindle motor is the result.
Further still, a ring element 112 that constitutes a retainer for the rotor 100 is fitted onto the end portion of the shaft 106 on the side opposite the rotor 100. In short, because the thrust bearing section 104; the radial bearing sections 108, 108; the through-hole 110a as well as the channels 110b, 110c that constitute the communicating passage 110; and the ring element 112 are arranged in the axial direction stacked along the same axis, they create an impediment to making the spindle motor slimmer.